Spinets, in general, are a class of minerals having the general formula A2+B23+O42−. Aluminum spinels include spinel, MgAl2O4, gahnite, ZnAl2O4, hercynite, FeAl2O4. Although spinels are found in nature, because of the increasing demand synthetic spinels are now being made by various methods.
Presently, spinels are used as windows and domes in high-speed missiles and pods. Although silicate glasses are suitable for most common window uses, they are too weak to be used in the latter applications. Furthermore, missile domes, pod windows, and other transparent armor must transmit light wavelengths up to 5 μm, whereas silicate glasses do not transmit well beyond wavelengths of 2 μm. Although zinc sulfside transmits to beyond 5 μm, it is too weak to be used as transparent armor. Magnesium aluminate spinel is a polycrystalline ceramic material that has excellent optical and mechanical properties. It transmits wavelengths of light from 0.2-5.5 μm and ballistic testing has shown that 0.25 inch of spinel has the same resistance as 2.5 inches of bullet proof glass. Spinel is therefore an excellent candidate for transparent armor in light vehicles and goggles or face shields on infantry helmets.
The synthesis of spinels of the type in consideration is well-known in the literature. In this regard, it is well established that an intimate mixture of the component materials is desirable in order to obtain a high conversion to spinel upon thermal treatment. There are many syntheses of magnesium aluminate spinels in the literature. The majority of these can be divided into three basic classes: coprecipitation of aluminum and magnesium salts; ball milling of magnesium and aluminum precursors; and wet mixing of alumina and magnesium compounds. Each of these process have inherent limitations which limits their ability on an industrial/commercial scale. For example, all of these methods suffer from a similar problem of residual inclusions, mainly of unconverted precursors, which cause undesired dispersion of lightwaves passing through the spinel.